central A/C unit is not cooling as usual

caliaJuly 23, 2011

Central A/C unit has not been cooling below 78 degrees, for 2 days. I set the thermostat to 74 last night, but the house temp did not go below 78. Today, I set to 77. The temp was 78, and up to 80 by late afternoon. It is not cooling down.

The fan is running outside, the thermostat shows correct day and time, etc. The filter was changed 2 weeks ago. I'll try to describe he indoor unit but I'm new to this... the large insulated tube is cold, the smaller one is warm. The largest tube (like a chimney stack?) on top of the unit feels cool, but not cold (it gets hot with the heater, so should I expect it to be cold with A/C?) The air coming from floor vents is cool, but not cold.

Admittedly, we are experiencing more extreme heat than we are used to, but I want to be sure that the A/C unit is working as it should, all considered.

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A cooler day today, A/C seems to be ok. :)

    Bookmark   July 24, 2011 at 8:57AM
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Keep in mind that air conditioning moves heat from inside to outside, at a specific rate as per the BTU capacity of the system. It's constantly fighting against the heat load that's generated inside the house by cooking, lights, use of appliances (hair dryer, etc.) and electronics (computers generate heat) ... and heat that radiates in through walls and roofs heated by sunshine, windows, doors, etc. Other variables are involved such as humidity levels (squeezing moisture out of the air requires removing heat but the effective temp of the air doesn't drop ... latent heat). Units are sized to exceed the heat load under "normal" conditions so the temp inside the house can be reduced. There's a "balance-point" at which the indoor temp holds steady, the heat load exactly matches what the system can pull out. If the heat load exceeds the unit's capacity, as may happen during extremes of weather, the indoor temp will increase, although it will still of course be cooler inside than out. Setting the thermostat cooler will not have an effect in that situation ... it won't make the system run "harder" to remove more heat*, the system is either ON and removing all the heat it can, or it's OFF and removing none.

A way to determine if the unit is functioning within normal limits is by checking the intake air temp (at the filter grill) and the output air (at the closest register). There should be a reduction in temp of about 16F to 20F ... which is the heat removed as air flows through. If the differential is less than 15F (more or less), then there may be a problem. Checked mine (5-ton, 60,000 BTU system) a few mins ago, 78F in, 57F out.

*Except maybe in the case of a two-speed system if it's running on low speed and setting the thermostat cooler triggers it to high speed ... but in such a situation the system would likely already be running on high speed.

    Bookmark   July 24, 2011 at 10:10AM
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I hope you don't mind that in addition to your accurate description, I'd like to add one more thing that might help some of the folks, since the outside temperatures in some locations are higher than normal this year.

Check the air coming from the condensing unit outside, it should be warmer than the outside air. That is the heat dadoes describes being removed from your home, in addition to that the compressor causes by compression. Units are designed and sold according to region. As the outside temperatures rise above the design temperature, the less the temperature differential on the condensing unit and the harder it becomes to transfer the picked up heat, into the outside air. This is one of the major causes for units not performing as people are accustomed to, when the temperatures in a region are much higher than those considered normal. Your unit may run longer or not cool as much as you are used to until maybe the evening hours, especially if in addition, the unit is exposed to the sunny side all day.

    Bookmark   July 24, 2011 at 9:37PM
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YES! I've sort of wondered about that. If say in Texas vs Kentucky, obviously Texas is a hotter climate, so a/c's installed there would be calculated for their climate? Here is Lexington it has been mid 90s and during the afternoon from say 2pm to perhaps 8pm, my AC runs NON STOP but ONLY if it's above 92 degrees outside and stays above 92 degrees for four hours or more. Blinds closed, black out curtains drawn and it stays very cool inside. I've noticed that (I keep the temp at 73) that it will go up a degree or two for a couple of hours starting at around 4pm if it's FULL sunshine outside. If it's very hazy but hot with filtered sunshine, it maintains the temp. My outside unit is on the south side with little to no shade. I checked the temp in the return and it was 74 degrees and the intake and the output of cool air was 57 degrees. A 17 degree difference. Interestingly enough, at night when I check it there's a 20 degree difference. Why I don't know.

    Bookmark   July 29, 2011 at 3:31AM
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On average, around 3pm to 4pm starts the hottest time of day and 3am to 4am is the coolest. There are some units that are actually designed to use the condensate to help cool the outside condensing coil. You can test this yourself with a garden hose and a slight spray onto the condensing coil and measure your temperature differences. For instance, on window units, the water is designed to sit in the bottom of the frame and by virtue of the "open refrigeration cycle" the air going through the condensing coil is cooler. Some fan blades have a ring connecting the tips of the blade. The ring runs through that water and flings it into the coil, helping increase efficiency. On hot humid days you can hear the blades hitting water.

If you can give your condensing unit shade without impeding air flow and allow the condensate to drain on the ground around the condensing unit, you can lower the air temp a degree or two.

    Bookmark   July 29, 2011 at 8:27AM
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